In conventional pulping operations, the raw cellulosic fibrous material, generally wood chips, is digested in a pulping liquor, generally known as a "white liquor". This latter term is used in this specification generally to refer to liquors containing dissolved sodium hydroxide. After the digestion step, the pulp is separated from spent pulping liquor, known as "black liquor".
In the normal kraft process, the cellulosic fibrous material, generally wood chips, is digested by heating with a white liquor containing sodium sulphide and sodium hydroxide to dissolve from the wood chips a substantial part of the hemicelluloses and lignin therein. The fibrous material so produced is separated from the resulting black liquor, by washing counter-current with water in a brown stock washing plant and, thereafter, may be passed to a bleaching process.
The black liquor is subjected to a series of operations in a recovery system. The black liquor first is concentrated by evaporation of water and the concentrated black liquor is burned in a furnace to yield a smelt containing mainly sodium carbonate and sodium sulphide. The smelt is dissolved in water to yield a raw green liquor which then is clarified. The dregs resulting from the clarification, generally consisting of insoluble salts of metal cations other than sodium and potassium, and of clear residues, are washed with water and discarded.
The clarified green liquor is causticized with slaked lime whereby the sodium carbonate is converted to sodium hydroxide and calcium carbonate is precipitated as a mud. The mud is washed with water and calcined to regenerate lime for further causticization. The causticized green liquor is then recycled as white liquor to the digester. The wash water from the dregs and the mud, generally referred to as "weak wash" or "weak white liquor", is usually used as water for dissolving the smelt.
Sulphur and sodium containing by-products from chlorine dioxide generation, purchased sodium sulphate, and elemental sulphur and/or sodium hydroxide or soda ash, are added to the recovery operation to provide make-up sodium and sulphur values to the system. Generally the sodium sulphate is added to the black liquor before it is fed to the furnace. The sodium and sulphur values in the furnace give sodium sulphide and sodium carbonate, the sodium carbonate being converted to sodium hydroxide on later causticization. In this manner, the sodium hydroxide and sodium sulphide content of the white liquor is maintained at the desired level.
During the past decade, and because of environmental concerns, considerable effort has been invested in improving the efficiencies of traditional pulp and paper manufacturing facilities in North America, and elsewhere, and minimizing the discharge of pollutants from such mills. In recent years, kraft pulp digesting processes employing what is commonly known as a modified continuous cooking (MCC) process have become popular. A digester system known as the Kamyr modified continuous cooking process digester system has been widely installed in pulp and paper manufacturing facilities throughout the world.
In a conventional pulping process, the lignin in the pulp must be separated from the cellulose and hemicellulose. However, in the process, the delignification reaction must be emphasized while the cellulose and hemicellulose degradation reactions must be minimized. Degraded cellulose and hemicellulose reduce the inherent strength qualities of the pulp for paper making. It is therefore important in a pulping process to obtain high selectivity. The term "high selectivity" in a pulping process means that the rate of the delignification reaction is high compared to the rates of reactions that degrade cellulose and hemicellulose. High selectivity in pulping makes it possible to produce a pulp with low residual lignin content prior to bleaching while maintaining good pulp strength by minimizing cellulose and hemicellulose degradation. By minimizing the lignin content of the pulp entering the bleach plant, less lignin has to be removed through the bleaching process, thus reducing bleach chemicals consumption and the contaminant content of the bleach plant effluent.
In the Kamyr MCC pulping process, the selectivity of the process compared to a conventional process has been improved by keeping the hydroxide concentration moderate and even throughout the cook, and keeping the dissolved lignin and sodium concentration as low as possible, which is especially important during the final phase of the cook.
In the Kaymr MCC process, the alkali charge is generally divided into three portions, one to the impregnation vessel, one to the trim circulation and one to the final part of the cook. In this manner, the hydroxide concentration is evened out throughout the process. Further, the final phase of the cook is carried out in countercurrent mode to lower the dissolved lignin concentration during this phase. In certain recent Kamyr MCC installations, a four-way split of the white liquor charge is made, with the three portions applied as discussed, and the fourth portion applied to the wash zone recirculation. However, the triple- or quadruple-divided alkali charge also has an undesired consequence--it lowers the sulphide concentration during the early stages of the cook where the sulfide is beneficial to the selectivity of the cook. This is because part of the white liquor is charged to the latter portion of the cook where sulphide content does not have any beneficial effect.
It is known that the selectivity of the modified Kamyr MCC process or other kraft delignification processes can be improved by increasing the sulphide concentration in the early stages of the cook.
Canadian Patent No. 923,256, Rapson et al disclose a process of recovery of spent chemicals from the pulping of cellulosic fibrous material, including the step of treating a smelt containing sodium sulphide, which comprises contacting the smelt with a solution of sodium hydroxide having a concentration of about 15 to 40% NaOH to dissolve sodium sulphide from the smelt. They obtain this high concentration sodium hydroxide solution by evaporating a sulfide-lean white liquor. The process is primarily directed to removing sodium chloride from the smelt. Sodium sulphide is preferentially dissolved in the sodium hydroxide solution, thereby separating sodium chloride. The process produces a single stream of white liquor for use in the pulping process. There is no mention of separate sulfide-lean and sulfide-rich white liquor streams which are directed to different phases, initial, bulk, and final, of the delignification procedure to thereby provide improved efficiencies in a pulping process.
Lukes et al., U.S. Pat. No. 3,909,344, describes a process primarily aimed at the removal of sodium chloride from the kraft pulping recovery cycle. To achieve this, an aqueous solution of smelt containing sodium carbonate, sodium sulfide and sodium chloride, obtained by dissolving the smelt in a solution of sodium carbonate and sodium chloride (neither of these solutions being a white liquor) is cooled to effect crystallization of the solid heptahydrate or decahydrate of sodium carbonate from the solution at temperatures around 25.degree. C. This hydrate is then separated from the mother liquor, dissolved and causticized. The mother liquor is then treated in a number of steps including evaporation and carbonation to separate the chloride and sulfide values from the liquor.